Rubber like behavior

Elastomers are polymeric substances with rubber-like behavior at ambient temperatures. This means they are more or less elastic, extensible, and flexible. They can be exfended by relafively small force and return to the original length (or near it) after the force is removed. Rubber-like behavior can be observed in plastics, but under different conditions, such as at elevated temperatures or in swollen state. These are not true elastomers, however. 

Elastomers, prepared by free-radical initiated copolymerization of ethyl acrylate with cellulose to several hundred percent extent of grafting of poly (ethyl acrylate) onto cellulose, exhibited rubber-like behavior and second-order transition temperatures. Cellulose-poly (ethyl acrylate) elastomers had transition temperatures below —35°C, about — 20°C, and below 5°C when measured in ethyl acetate, dry air, and water, respectively (43, 44). 

The elastomers exhibited rubber-like behavior. From an examination of electron photomicrographs of cross sections of the elastomers, the fibrillar structure of the cellulose fibers apparently formed a network, and poly (ethyl acrylate) was distributed uniformly among the fibrils. The rigid crystalline regions of the cellulose fibers apparently stabilized the amorphous, grafted poly (ethyl acrylate) to determine the mechanical properties of the elastomers (43, 44). For example, typical elastic recovery properties for these elastomers are shown in Table X. 

Tg, the force—extension curve constructed in this manner for the first contraction displays a rubber-like behavior at low strains as the amorphous regions become oriented 42). 

Random copolymerization of aliphatic monomers develops elastomeric or rubbery nylons by reducing interchain attractions and lowering the decree of crystallinity by destruction of chain r ularity. Random copolymerization of ahphatic monomers generally decreases both meehanieal and thermal properties of nylorts but, in some cases, it allows rubber-like behavior in the copolymer. 

Fig. 5. Stress-strain curves at various temperatures (increasing from a to e) (a) low extensibility followed by brittle fracture at the lowest temperature (b) localized yielding followed by fracture (c) necking and cold drawing (d) homogeneous deformation with indistinct yield (e) rubber-like behavior.

As it was noted above, the cluster model [18,23] explains two more features of glassy polymers behavior on cold flow plateau. An experimentally observed high values are due to high values v j, which are about of order larger than Vj [23] and glassy polymer rubber-like behavior on the indieated plateau is due to loosely packed matrix rubber-like state. 

Minor morphological differences, as between MR and NATSYN show up by their thermomechanical responses. Work on polymer chain conformations in bulk, 1968-to date, especially on fluorinated acrylics with B. McGarvay and W. Lee, on elastomeric dienes with H. Mark and K. Sato, and of entanglements in GRS witn R. Meyers, followed. These experiences were reviewed, with emphasis on the structural factors in rubber-like behavior at all phases of extension, in a chapter coauthored by T. L. Smith, on the Rupture of Elastomers in Liebowitz s treatise "Fracture , while later analyses focused on the effect of fillers in elastomers, the role of interfaces in composites, failure mechanisms in plastics as a function of morphology, and on resulting lessons for polymer engineering. 

Rubber-like behavior also constitutes a mechanical type of shape memory as does the process of SIM formation. But rubber-like behavior is characteristic of a fully martensitic structure whereas superelastic behavior is associated with formation of martensite under stress. These two types of behavior collectively fall in the category of pseudoelasticity , but one should use care in the interest of preciseness. Broadly speaking, Olander s (1932) report of rubber-like behavior in Au-Cd was the first indication of the existence of a shape-memory effect. It is ironic that even today, 60 years later, the origin of rubber-like behavior remains obscure. 

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